The present invention relates to volatile liquid storage tanks and more particularly to such tanks having apparatus for reducing vapor generation when the tank is filling from the upper or top side of the tank. These tanks could include above and below ground petroleum fuel tanks, chemical storage tanks, mobile liquid storage containers, and the like. Although the focus of this description will be on below ground fuel storage tanks, the invention has application with other tank types as well.
Riser pipes are used as the main liquid fill lines for fuel storage tanks located below ground at gasoline service stations and the like. The riser pipe is vertically oriented, has its upper end located in a manhole, and its lower end opening towards the inside of the below-ground storage tank. Because it is important to fill fuel storage tanks from the bottom of the storage chamber, drop tubes are inserted through the riser and have their outlet ends positioned near the bottom of the storage chambers. Risers and drop tubes must be dimensioned to accommodate a flow of approximately 340 gallons per minute.
Drop tubes function to confine the inflow of liquid until the inflow exits the bottom of the tube near the bottom of the tank. This action serves to essentially fill the tank from the bottom so that liquid is released into the tank under or below the liquid level already stored in the tank. This action reduces turbulence and unwanted vapor generation above the stored liquid level. However, it is known that vapors, a vapor/air mixture, or other or non-liquid fluids (hereafter collectively and severally "vapor") substantially fill the drop tube above the stored liquid level.
In addition, the delivery hose coupled between the tanker truck fill coupler and the top of the riser and the riser itself are substantially completely filled with air when the filling operation begins.
The operator initiates liquid fill by simultaneously opening the tanker foot valve and fill coupler valve which causes fill liquid to drain by gravity from the chosen tanker compartment into the delivery hose. Because of the high rate of flow through about a four inch hose, the flow acts as a rushing wall forcing the air in the hose and riser down into the drop tube. Because the volume initially occupied by the air in the hose riser and drop tube is reduced to the shrinking space in the drop tube, the vapor pressure in the tube rises rapidly. As filling continues, the trapper air and vapor first slow the flow rate of fill liquid util most of the air vapor is carried down and exits the bottom of the tube where it bubbles up toward the stored liquid surface causing turbulence at the surface and generation of further vapor content which increases the fuel density of the vapor above the liquid surface in the tank. As the tank fills, vapors are drawn out of the top of the tank to equalize vapor pressure and are recovered by recycling them back to the top of the tanker truck storage compartments. If the vapor pressures in the storage and fill system rise beyond permissible limits, safety valves vent the vapor to the atmosphere.
Accordingly, there is a general desire to fill liquid storage tanks in such a way as to reduce or avoid excessive vapor generation and reduce the vapor pressure that restricts early flow of fill liquid.
Attempts have been made to reduce the surface turbulence as described above. For example, UK Patent GB 2,301,347, incorporated herein by reference, describes a below ground gasoline storage tank having an elongated "piccolo pipe" with a series of openings along its length mounted longitudinally within the drop tube and having an upper end communicating with a junction piece at the top of the drop tube. This junction is fluidly connected to the vent stack at the manhole cover via a further connector pipe which attaches in fluid connection to the top of piccolo pipe.
During delivery of liquid product to the tank, according to U.K. patent GB 2,301,347, as the product travels down the tanker hose, the displaced air travels down into internal regions of the drop tube, and into the apertures of the piccolo pipe. Once inside the piccolo pipe, the air/vapor moves up the pipe, into the vent stack and is eventually vented without coming into contact with the liquid stored within the tank.
The foregoing apparatus is not free of technical, cost, and installation problems since it requires a junction and new piping to be laid below ground running from the top of the manhole/riser area to the bottom of the tank venting system. In addition, with this arrangement, high pressure vapors fed from the piccolo pipe are vented to the atmosphere instead of being returned to the tanker fueling compartments as is the practice in the United States. Further, this prior art system requires float valves to shut off the piccolo pipe when the liquid rises to a predetermined tank level in order to prevent liquid overfill or spills. These floats may bind in the tube or otherwise wear out resulting in liquid spills and/or requiring pulling and maintenance of the drop tube.